1. Field of the Invention
The present invention relates to a liquid ejector which uses an ultrasonic wave to eject droplets of ink from an ink liquid surface, and a printing apparatus such as an ink jet printer which employs such a liquid ejector to print characters and images on recording paper.
2. Description of the Background Art
In the field of printing apparatuses, an ink jet head which uses an ultrasonic wave to eject ink from a nozzle has conventionally been known as a liquid ejector. For example, K. A. Krause, "Focusing Ink Jet Head", IBM Technical Disclosure Bulletin, Vol. 16, No. 4, 1973, p. 1168 discloses an ink jet head which emits a jet of ink from a nozzle provided adjacent the focal point of the ultrasonic wave. This ink jet head is designed such that the ultrasonic wave emitted from a piezoelectric vibrator mounted on the rear surface of a member having a concave surface which contacts ink is refracted at the concave surface to propagate in the ink while being focused.
A liquid drop emitter which employs a curved crystal having a concave surface to focus an ultrasonic beam from the liquid ejector is disclosed in U.S. Pat. No. 4,308,547. A driving method for emitting droplets one by one is applied to the liquid drop emitter. The liquid drop emitter is designed to intermittently apply a drive signal at the resonant frequency of the crystal so that the number of drive signals intermittently applied equals the number of emitted droplets.
FIG. 15 is a cross-sectional view of a liquid drop emitter disclosed in Japanese Patent Application Laid-Open No. 63-166545 (1988) which uses the liquid drop emitting technique disclosed in the above referenced U.S. patent. In FIG. 15, the reference numeral 1 designates ink; 2 designates a liquid surface of the ink 1; 3 designates a substrate mounted in an ink reservoir filled with the ink 1 for directly transmitting an ultrasonic wave into the ink 1; 4 designates a vibrator mounted on the bottom surface of the substrate 3; 5 designates a lead for electrically sending a drive signal to the vibrator 4; 6 designates an RF controller for outputting the drive signal to be sent through the lead 5; and 7 designates a tube for supplying the ink 1 to retain the ink liquid surface 2 in position. The substrate 3 includes an acoustic lens 3a having a curvature such that the focal point of the ultrasonic wave emitted from the substrate 3 is adjusted to be at the ink liquid surface 2. FIG. 16 is a schematic view of the liquid drop emitter of FIG. 15 which shows that the ultrasonic wave is focused by the acoustic lens 3a. Like reference numerals are used in FIG. 16 to designate elements identical with or corresponding to those of FIG. 15.
A high-frequency drive signal (referred to hereinafter as a burst signal) which is AM modulated by a pulse signal is applied from the RF controller 6 through the lead 5 to the vibrator 4 of FIG. 15. The vibrator 4 vibrates in the thickness direction at the high frequency only in the presence of the high frequency in the burst signal, to generate an ultrasonic wave 8 and transmit the ultrasonic wave 8 to the substrate 3. The ultrasonic wave 8 transmitted to the substrate 3 propagates in the substrate 3, and is partially refracted by the acoustic lens 3a to become an ultrasonic beam 9 propagating in the ink 1. The ultrasonic beam 9 is focused on the ink liquid surface 2, and ink droplets 11 are emitted from a focal point 10 at which the pressure is increased by the ultrasonic beam 9.
Controlled emission of the ink droplets 11 one by one is achieved by applying a high-frequency signal to the vibrator 4 for a short time period each time the ink droplet emission is required. FIGS. 17A through 17C are timing, charts illustrating the application of the high-frequency signal. The high-frequency signal is a radio-frequency signal (RF signal) at the resonant frequency of the vibrator 4 and is shown in FIG. 17A. For the application of the high-frequency signal for a predetermined time period for each requirement of the droplet emission, the RF signal is AM modulated by a gate signal (FIG. 17B) which is a pulse signal having a period Ta and a pulse width Tb to produce the burst signal shown in FIG. 17C. The application of the burst signal to the vibrator 4 causes an ultrasonic radiation pressure to act like pulses upon the focal point 10 to allow the one-by-one droplet emission.
FIGS. 18A through 18E are cross-sectional views of the ink liquid surface 2 at different times for illustration of the formation of a droplet. FIG. 18A shows the initial state wherein the ink liquid surface 2 of the ink 1 is flat since no ultrasonic radiation pressure acts upon the ink liquid surface 2. As the ultrasonic radiation pressure acts upon the ink liquid surface 2, the ink surface 2 is raised into a mound as shown in FIG. 18B. Thereafter, part of the mound starts separating in the vertical direction as shown in FIG. 18C, resulting in the separation of a droplet as shown in FIG. 18D. Then, the ink liquid surface 2 returns to its initial state wherein it has no mound but is flat because of its surface tension as shown in FIG. 18E. The time T0 required for a series of operations shown in FIGS. 18A through 18E is determined by the surface tension and density of the liquid (ink 1), the diameter of the focal spot, and the like. Thus, the print head is designed so that the period Ta of the pulse signal is greater than the time T0 for the one-by-one droplet emission. The details of the above described principle is described in S. A. Elrod et al., "Nozzleless droplet formation with focused acoustic beams", J. Appl. Phys. 65(9), May 1, 1989.
A method of varying the size of droplets by modulating the RF signal is also disclosed in Japanese Patent Application Laid-Open No. 63-166545. The method mainly includes processes for (1) varying the time duration (pulse width Tb) of the RF signal, (2) varying the amplitude of the RF signal, and (3) varying the frequency of the RF signal. The processes (1) to (3) are used alone or in combination to control the resolution of a printer.
FIG. 19 shows the printer disclosed in the above referenced patent application. In FIG. 19, the reference numeral 20 designates recording paper 20, and 21 designates rollers for feeding the recording paper 20. Like reference numerals are used in FIG. 19 to designate elements identical with or corresponding to those of FIG. 15. The printer of FIG. 19 comprises a print head similar to that shown in FIG. 16, and is adapted such that a plurality of fine ink droplets 11 of the same diameter emitted one by one from the print head are deposited on the recording paper 20 at the same position. A spot diameter Sd recorded on the recording paper 20 is varied as shown in FIG. 20 to allow the gray scale representation. A pixel is shown in FIG. 20 as a square region enclosed by dotted lines.
An ink jet head having a nozzle at an ink liquid surface and jetting droplets from an opening of the nozzle is disclosed in Japanese Patent Application Laid-Open No. 2-303849 (1990). The burst signal is used as the drive signal for driving the ink jet head. The amount of ink emitted from the ink jet head is controlled by varying the time duration for which the RF signal appears in the burst signal. The arrangement disclosed in this reference establishes a longer time duration of the RF signal for emission of a greater amount of ink. This causes the prolonged application of the ultrasonic radiation pressure to the nozzle opening. As a result, the droplets are considered to be emitted in the form of a spray from the nozzle opening.
The background art liquid ejector as shown in FIG. 15 which requires no nozzle is advantageous in eliminating the problem of clogging with ink. However, the liquid ejector of FIG. 15 must establish a high frequency of the RF signal for emission of fine droplets since a major factor which determines the droplet diameter depends on the focal spot diameter of the ultrasonic beam 9. As is observed by S. A. Elrod et al., the focal spot diameter of an acoustic lens having a focal length which is generally equal to the opening diameter is equal to the ultrasonic wavelength in the ink 1. For example, the velocity of sound in typical water-based ink is about 1500 m/s. Thus, in order to form droplets having a diameter of about 3 .mu.m, the frequency of the RF signal must be 500 MHz to provide the wavelength of 3 .mu.m. To handle such a high-frequency ultrasonic wave, a drive circuit is required to have a complicated structure and high-accuracy constituents, resulting in a very costly liquid ejector. Further, the requirement for the finishing accuracy of the surfaces of the acoustic lens 3b of the liquid ejector and the level accuracy of the ink liquid surface 2 to be equal to or greater than the accuracy of the wavelength makes it difficult to produce the droplet emitter.
Furthermore, the fine ink droplets are recorded by depositing one droplet over another to vary the spot diameter Sd on the recording paper 20, as shown in FIG. 20. Thus, there is a need to allow for time it takes to emit a required number of droplets for recording the spot of a maximum diameter, requiring much time for recording. Liquid ejectors other than that disclosed in Japanese Patent Application Laid-Open No. 63-166545 are believed to be controlled under stable conditions only within a limit which is twice the droplet diameter and to be difficult to represent the gray scale only by varying the droplet diameter.
The method of jetting the droplets from the nozzle opening of the liquid ejector as disclosed in Japanese Patent Application Laid-Open No. 2-303849 involves the need for a nozzle plate having a fine opening in order to reduce the size of the droplet diameter. Further, since the time duration of the RF signal is increased for emission of more ink, the droplets are emitted in the form of a spray from the nozzle opening. This causes random diameters of the droplets to present difficulty in forming a high-definition image.